Shared linearity maintenance in power amplifiers

ABSTRACT

In some embodiments, a circuit includes a power amplifier including an input terminal configured to receive an input signal and an output terminal to provide an RF voltage, the output terminal coupled to a load, a current sensor configured to sense the current drawn by the power amplifier and provide a first sensor output signal dependent upon current consumption when the current exceeds a predetermined current threshold, a voltage sensor configured to sense the output power of the power amplifier and provide a second sensor output signal when the RF voltage during up ramp falls below a predetermined threshold voltage, and a summing circuit configured to receive the first and second sensor output signals and provide a feedback signal including a combination of a power dependent contribution and either of a voltage dependent contribution or a current dependent contribution.

RELATED APPLICATIONS

This patent application claims the benefit of priority, under 35 U.S.C.Section 119(e), to U.S. Provisional Patent Application Ser. No.60/863,109, filed on Oct. 26, 2006, which is incorporated herein byreference.

TECHNICAL FIELD

Embodiments described herein relate generally to power amplifiers andmore particularly, to power amplifiers for driving antennas of varyingload impedance.

BACKGROUND

Global System for Mobile Communications (GSM) is one of the standardsused for mobile phones. Gaussian Minimum Shift Keying (GMSK) is a typeof continuous-phase frequency-shift keying used in GSM. Enhanced Datarate for GSM Evolution (EDGE) is a digital mobile technology used inconjunction with GSM to provide packet-switched applications such asinternet connection. EDGE additionally uses 8 phase-shift keying (8PSK)as part of the modulation and coding scheme. Mobile handsets using suchtechnologies use power amplifiers and derive power from a battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic of a circuit for maintaining sharedlinearity in power amplifiers, according to some embodiment of theinvention.

FIG. 2 shows a flow diagram of a method to maintaining shared linearityin power amplifiers, according to some embodiment of the invention.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingsthat show, by way of illustration, specific details and embodiments inwhich the invention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. Other embodiments may be utilized and structural, logical,and electrical changes may be made without departing from the scope ofthe invention. The various embodiments are not necessarily mutuallyexclusive, as some embodiments can be combined with one or more otherembodiments to form new embodiments.

Amplifier linearity is a fundamental requirement for the operation ofEDGE technology in mobile handsets. Any amplitude distortion of thesignal envelope produces two unacceptable phenomena. Firstly, thespectrum of the signal is widened (also known as spectral re-growth).This widening effect can cause the signal to fail the prescribedmodulation mask, a requirement set by the GSM standards to preventinterference to neighboring channels. Secondly, a simple distortion ofthe modulation constellation results in a lowered signal to noise ratioat the receiver. The GSM standards define an error vector magnitude(EVM) specification, which is a measure of the difference between thetransmitted signal and an ideal one. In practice, imperfections in themodulator, other transmitter stages and non-linearity in the poweramplifier can consume a significant fraction of the EVM budget.

EDGE standard is based on a modified 8PSK modulation scheme. As a resultof baseband filtering the final modulation signal is also amplitudemodulated which means a non-constant envelope. Consequently, due to thepresence of a non-constant envelope, the spectrum at the output of poweramplifier strongly depends on the linearity of the power amplifier used.Hence, it is desired to have 8PSK power amplifiers optimized in order tomeet the stringent linearity requirements.

There are two classes of 8PSK power amplifiers. The first class supportsEnvelope Elimination and Restoration (EER) architectures (e.g. LargeSignal Polar Loop, Polar Modulator). Here, the RF input signal is phasemodulated and the amplitude modulation is injected via the poweramplifier supply voltage or the power amplifier bias voltage (to do thisthe final 8PSK signal after Base Band filtering must be separated in AMand PM using a “polar” approach). Power amplifiers in this first classis non-linear with respect to RF input signal but should have a linearrelationship between RF voltage and bias/supply voltage. Additionally,if the power amplifier is not linear at this point a closed loop AMcontrol or AM pre-distortion can be used. The second class of poweramplifiers is linear with respect to RF input signal. Additionally, theRF input signal is amplitude modulated and phase modulated.

The linearity performance of a linear power amplifier can stronglydepend on the load impedance of the antenna. A non-sufficient linearityperformance is indicated by bad EVM and/or bad spectrum due tomodulation Adjacent Channel Power Ratio (ACPR). Depending on load, phasedifference effects are responsible for degradation of spectrum. For loadstates that result in a high impedance (typically seen at the last stageof power amplifiers), the RF voltage is clipped thereby resulting in anon-linear behavior. In the low impedance case, the input impedance atthe last stage of the power amplifier is affected in such a way thatmore power is delivered to last stage thereby resulting in currentclipping (negative swing effect). This effect could be improved byincreasing the quiescent current, but, the improvement is achieved atthe expense of efficiency.

Low and high impedance load states are potential sources for significantACPR degradation. Consequently, understanding of the low and highimpedance load states is important and any measure intended to improvelinearity must address both effects of low and high impedance loadstates.

In some embodiments, an 8PSK linear power amplifier is designed to copewith a certain voltage standing wave ratio (VSWR). For severe load(impedance) mismatch the ACPR and EVM performance gets worse. In someembodiments, in order to maintain sufficient ACPR and EVM at severe loadmismatch, the forward power is reduced.

In some embodiments, either change of load impedance or mitigation ofload VSWR is achieved. The change of load impedance is achieved by loadswitching. The VSWR mitigation is realized by using hybrid architecture.The hybrid approach is effective to increase load insensitivity but onlyworks up to a particular VSWR. Beyond this particular VSWR, the spectrumis corrupted. Load switching technique in handsets can be complex andexpensive to achieve. The alternate approach relies on reduction ofoutput power. One approach used is to take the reflected signal as anindication for mismatch. However, the linearity degradation also dependson the phase of the reflected signal from the load. Since, only themagnitude of reflected signal is measured, the output power is alsoreduced for different load phases where it is actually not necessary.Consequently, Total Radiated Power (TRP) performance is sacrificed. TRPis the measure of the mobile device's radiated output power. TRP is afunction of the output power of the power amplifier, the antenna'sradiation efficiency and the power amplifier's sensitivity to antennamismatch (impedance mismatch). Improvements in TRP can increase networkefficiency, network coverage and data throughput rates while alsoreducing the frequency of dropped calls.

FIG. 1 illustrates a schematic of a circuit 100 for maintaining sharedlinearity in power amplifiers, according to some embodiment of theinvention. Circuit 100 includes a front end circuit 110, and anamplifier circuit 120 coupled to an antenna 132 and impedance 134. Frontend circuit 110 includes a controller 112. Amplifier circuit 120includes a power amplifier 122, coupler 124, power detector (voltagesensor) 126, current/saturation sensor 128, and summing circuit 130.Power amplifier 122 receives input power PIN and amplifies PIN andoutputs a power P_(OUT).

In some embodiments, since load state is not known, the mobile SW cannotinduce the power reduction. Therefore, it is required that thetransceiver module has built-in linearity sensors intended to recognizeload states which cause spectral degradation. In some embodiments, twosensors are incorporated into the amplifier circuit 120 namely thecurrent/saturation sensor 128 and power detector (voltage sensor) 126.The current/saturation sensor 128 is needed to recognize the lowimpedance case (below matched impedance) whereas the power detector(voltage sensor) 126 is relevant for high impedance case (above matchedimpedance). The current/saturation sensor 128 measures the DC current ofpower amplifier 122. High current consumption is an indication of thepresence of a low impedance load. When the DC current exceeds athreshold the current/saturation sensor 128 is activated and generates avoltage which depends on current consumption. The power detector(voltage sensor) 126 measures the minimum RF voltage during up ramp.When the RF voltage falls below a given threshold voltage V_(RF,min) thesensor is activated.

In some embodiments, a Shared Linearity Maintenance scheme is used aspart of a power control loop (hereafter called APC) in the poweramplifier. In some embodiments, a closed loop power control for 8PSK isused and it includes a coupler 124, a power detector (not shown) andcontroller 112. In some embodiments, the controller is incorporated inan RF transceiver and the coupler 124 and the power detector are insidethe power amplifier 122.

In some embodiments, both sensors generate a voltage which eitherdepends either on current or minimum RF voltage. The current/saturationsensor 128 generates a voltage which is proportional to DC currentconsumption. The power detector (voltage sensor) 126 generates a voltagewhich is proportional to a minimum RF voltage. At any given time, onlyone of the current/saturation sensor 128 or the power detector (voltagesensor) 126 is active. The voltages generated by current/saturationsensor 128 or voltage sensor 126 are termed as sensor voltages.

The principle of Shared Linearity Maintenance relies on analog feedback.In some embodiments, the sensor voltage is added to the power detectorvoltage. The power detector voltage itself is output power dependent andprovided by power detector. In some embodiments, the total V_(det)signal is used by APC and therefore fed back to a transceiver. In someembodiments, the transceiver includes the controller 112. Thecurrent/saturation dependent contribution of V_(det) is zero as long asthe DC current consumption is smaller than the current threshold lip orthe RF voltage is more than min RF voltage V_(RF,min).

In some embodiments, in order to meet the linearity requirement undermismatch conditions, the forward power must be reduced to increase powerback-off (which would result in more margins being needed for mismatch).In some embodiments, the power control loop tries to achieve thedetector voltage which was phased during production. To reduce theforward power the difference between target detector voltage anddetector voltage corresponding to DC current limit or min RF voltagelimit (reduced forward power) must be provided by sensor circuit. Insome embodiments, as a result of the added voltage, the power controlloop presumes that a higher forward power (“loop fooling”) is presentand the same V_(det) level results in a lower forward power.

In some embodiments, the current and voltage thresholds areprogrammable. This provides for more flexibility for handsetmanufacturers. In some embodiments, the thresholds can be optimizeddepending on a particular antenna characteristic. In some embodiments,the programming can be done via a serial peripheral interface (SPI)available in the amplifier circuit. In some embodiments, availability ofsoft current/voltage limiting function achieves good transient spectrumperformance. In some embodiments, the output power is only reduced ifreally necessary and thereby resulting in better radiation performanceof the handset.

In some embodiments, the invention combines the advantages of powercontrol and current/RF voltage control loop (hybrid loop). This isachieved by providing a loop feedback signal which comprises both apower-dependent contribution and either one of a current dependentcontribution or a voltage dependent contribution provided by using thevoltage sensor 126 or current sensor 128. In some embodiments, atmoderate VSWR, the power control loop allows for greater output poweraccuracy and at severe load mismatch, the forward power is reduced inorder to maintain good linearity.

In some embodiments, a conventional APC design can be re-used byproviding additional hardware provided within the power amplifier.

Some embodiments of a method 200 for improving the operation of anamplifier are shown in FIG. 2. In a first of a series of actions 210,the circuit is sensing a dc current drawn by an amplifier coupled toreceive an input signal and deliver an output voltage to a load. Atblock 220, method 200 includes a further action of sensing the minimumoutput voltage. At block 230, method 200 includes the action ofcomparing the dc current drawn by the amplifier to a predeterminedthreshold current. At block 240, method 200 includes the action ofcomparing the minimum output voltage to a predetermined minimum outputvoltage. At block 250, method 200 includes the action of providing afirst feedback signal if the dc current exceeds the current threshold,the amplitude of the first feedback signal determined by the amount thatthe DC current exceeds the threshold current.

At block 260, the action is providing a second feedback signal if the dccurrent is less than the current threshold and if the output voltage isless than a predetermined minimum output voltage, the magnitude of thesecond feedback signal determined by the amount that the predeterminedminimum output voltage exceeds the minimum output voltage.

At block 270, the action is combining one of the first feedback signalor the second feedback signal with a power-dependent contribution signalreceived from an output power sensor.

At block 280, the action is generating a hybrid feedback signal based onthe power-dependent contribution and either one of the first feedbacksignal and the second feedback signal.

The accompanying drawings that form a part hereof show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter may be practiced. The embodiments illustrated aredescribed in sufficient detail to enable those skilled in the art topractice the teachings disclosed herein. Other embodiments may beutilized and derived therefrom, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. This Detailed Description, therefore, is not to betaken in a limiting sense, and the scope of various embodiments isdefined only by the appended claims, along with the full range ofequivalents to which such claims are entitled.

Such embodiments of the inventive subject matter may be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations or variations of various embodiments.

Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description. In the previous discussion andin the claims, the terms “including” and “comprising” are used in anopen-ended fashion, and thus should be interpreted to mean “including,but not limited to . . . ”.

The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b), requiring an abstract that will allow the reader to quicklyascertain the nature of the technical disclosure. It is submitted withthe understanding that it will not be used to interpret or limit thescope or meaning of the claims. In addition, in the foregoing DetailedDescription, it can be seen that various features are grouped togetherin a single embodiment for the purpose of streamlining the disclosure.This method of disclosure is not to be interpreted as reflecting anintention that the claimed embodiments require more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive subject matter lies in less than all features of asingle disclosed embodiment. Thus the following claims are herebyincorporated into the Detailed Description, with each claim standing onits own as a separate embodiment.

1. A circuit; comprising: a power amplifier including an input terminalconfigured to receive an input signal and an output terminal to providean RF voltage, the output terminal coupled to a load; a current sensorconfigured to sense the current drawn by the power amplifier and providea first sensor output signal dependent upon current consumption when thecurrent exceeds a predetermined current threshold; a voltage sensorconfigured to sense the output power of the power amplifier and providea second sensor output signal when the RF voltage during up ramp fallsbelow a predetermined threshold voltage; and a summing circuitconfigured to receive the first and second sensor output signals andprovide a feedback signal including a combination of a power dependentcontribution and either of a voltage dependent contribution or a currentdependent contribution.
 2. The circuit of claim 1, wherein the poweramplifier is coupled to an external power controller using a serialperipheral interface (SPI).
 3. The circuit of claim 1, furthercomprising a serial peripheral interface adapted to receive a signalcorresponding to a desired predetermined current threshold.
 4. Thecircuit of claim 1, further comprising a serial peripheral interfaceadapted to receive a signal corresponding to a desired predeterminedthreshold voltage.
 5. A system, comprising: a power amplifier moduleincluding a current sensor to sense the current drawn by the amplifierand providing a first sensor output signal dependent upon currentconsumption when the current exceeds a predetermined current threshold,a voltage sensor to sense the power output of the amplifier and producea second sensor output signal when the RF voltage during up ramp fallsbelow a predetermined voltage threshold, and a summing circuitconfigured to receive the first and second sensor output signals andprovide a hybrid feedback signal including a combination of a powerdependent contribution and either of a voltage dependent contribution ora current dependent contribution; and an automatic power controllercoupled to the power amplifier module and configured to receive thehybrid feedback signal having a combination of a power dependentcontribution and either of a voltage dependent contribution or a currentdependent contribution.
 6. The system of claim 5, wherein the automaticpower controller is coupled to the power amplifier module using a serialperipheral interface.
 7. The system of claim 5, wherein the hybridfeedback signal is received by a front-end circuit including theautomatic power controller.
 8. The system of claim 6, wherein thepredetermined current threshold is set via the serial peripheralinterface.
 9. The system of claim 6, wherein the predetermined thresholdvoltage is set via the serial peripheral interface.
 10. The system ofclaim 5, further comprising an antenna coupled to the power amplifier.11. A method comprising: providing a first feedback signal for anamplifier connected to drive a load, the first feedback signal providinga signal having a magnitude dependent upon the load voltage when theload voltage falls below a predetermined voltage threshold; providing asecond feedback signal for the amplifier, the second feedback signalhaving a magnitude dependent upon the current drawn to power theamplifier when the current drawn exceeds a predetermined currentthreshold; and adding either one of the first feedback signal and secondfeedback signal with a power dependent contribution signal and providingthe combined signal as a hybrid feedback input to the amplifier.
 12. Themethod of claim 11, wherein the predetermined current threshold and thepredetermined voltage threshold is set via a serial peripheralinterface.
 13. The method of claim 12, wherein the predetermined currentthreshold and the predetermined voltage threshold is set using anautomatic power controller.
 14. A method, comprising: sensing a dccurrent drawn by an amplifier configured to receive an input signal anddeliver an output voltage to a load; sensing an output voltage deliveredto the load by the amplifier; comparing the dc current drawn by theamplifier to a predetermined threshold current; comparing the outputvoltage to a predetermined minimum output voltage; providing a firstfeedback signal if the dc current exceeds the current threshold, theamplitude of the first feedback signal determined by the amount that thedc current exceeds the threshold current; providing a second feedbacksignal if the dc current is less than the current threshold and if theoutput voltage is less than a predetermined minimum output voltage, themagnitude of the second feedback signal determined by the amount thatthe predetermined minimum output voltage exceeds the output voltage; andadding either one of the first feedback signal and second feedbacksignal with a power dependent contribution signal; and generating ahybrid feedback input signal to the amplifier based on the powerdependent contribution and either one of the first feedback signal andthe second feedback signal.
 15. The method of claim 14, wherein thepredetermined current threshold and the predetermined voltage thresholdis set via a serial peripheral interface.
 16. The method of claim 15,wherein the predetermined current threshold and the predeterminedvoltage threshold is set using an automatic power controller.